by Chip Walter
Bolk’s concepts weren’t universally accepted at the time, but as Gould later pointed out, he was clearly on to something. Because we are born with skullcaps that are soft and in pieces, we can not only bring larger brains into the world, but also grow them larger once they have arrived. When a chimp is born, its brain is already 40 percent of the size it will be when it is fully grown. And while it continues to develop after birth, it soon reaches its full size. We, on the other hand, are born with a brain that is less than a quarter of its full size (23 percent, to be exact). During the first three years of life it trebles in size. But that still leaves nearly a third more brain growth ahead of us, something that continues into early adulthood.
An interesting aspect of neoteny is that it doesn’t change existing gene expressions, it simply postpones them. In our case by delaying or discarding the expression of certain genes, it drew a new sort of creature out of the womb into the real world. And when it did, it radically changed the way our kind subsequently evolved.
Are We Infant Apes?
During the first half of the twentieth century, several scientists added to Bolk’s original list of human neotenous traits. Evolutionary biologist Stephen Jay Gould compiled an expanded list in his book Ontogeny and Phylogeny, published in 1977. The general consensus among scientists now is that Bolk’s observance of neoteny was accurate, but his explanations for why and how it came about were not. (He believed it was due to a kind of endrocrine-induced retardation.) But it is obvious that we retain many features seen in infant and toddler apes in human adulthood, and the fossil record reveals in Australopithecus africanus, Homo erectus, Homo habilis, and Homo sapiens a progressive retention of juvenile traits. For example:
• Flat-faced orthognathy, the phenomenon that makes human brows less sloped and human jaws less pronounced than the jaws and brows of mature apes, but quite similar to the faces of infant apes.
• The lack of body hair. Newborn and young apes have less of it.
• Ear formations. Baby apes have ears that look more like ours.
• The central position of foramen magnum. This shifts backward as apes grow older.
• High relative brain weight. Toddler apes have a brain-to-body-weight ratio closer to ours than grown apes do.
• Persistence of cranial sutures. Other primates are born with sutures, but they close up much more quickly than ours do.
• Structure of hand and foot. Fetal apes actually have a big toe and foot shaped much more like human ones. Apes’ curled big toes develop as they grow to adulthood.
• Absence of bony brow ridges. Young gorillas and chimps don’t have bony brow ridges. They develop them later in life.
• Absence of cranial crests.
• Thinness of skull bones. We have hard heads, but they aren’t as hard as the heads of other primates.
• Wider head.
• Smaller teeth.
• Later eruption of teeth. This is another way of saying that we remain toothless longer and in this way resemble apes, who are born without teeth.
• Prolonged period of infantile dependency. We are babies longer than other primates.
• Longer life span. Another way of saying we remain young longer.
• Prolongation of fetal growth rate. Humans remain in the womb longer.
Neoteny was the evolutionary equivalent of the wheel or fire. Once our predecessors could walk upright, grow increasingly intelligent, and still manage to survive birth, whole new worlds opened up, and whole new evolutionary forces fell into place. We were mobile and could range across the savanna, hunt and forage by foot far more quickly than our knuckle-walking cousins. We could apply our increasing intelligence to the problems we faced and not be stymied, at least for the time being, by a brain restricted by the size of the birth canal. And above all, because we now spent so much time developing outside the womb, we were enriched by the world and experiences around us, which made us smarter, more adaptable, and more individual. We benefited from the sights, smells, sounds, and relationships of early life, while our brains were still compliant—embryonic—enough to react to these experiences and learn from them. We were coming into the world less hard-wired, less DNA-driven, and more impressionable than any other creature.
Anthropologist W. H. Krogman put it this way: “This long-drawn-out growth period is distinctively human; it makes of man a learning, rather than a purely instinctive animal. Man is programmed to learn to behave, rather than to react to an imprinted determinative instinctual code.”10 In other words, it made us not simply capable of learning—a dog or a mouse is capable of learning—but adapted for learning, dealing with change, and changing still further in reaction to that change. Our ancestors now had brains that could better shape themselves to the world around them, rather than be restrained by the strict marching orders of their genes. And all of this pushed our ancestors more closely toward being human. But our youthfulness, and the helplessness that accompanied it, would have still further social implications.
…
Most mammals are born far better prepared to handle the challenges of life than we are. At birth a wildebeest is up and running with the herd within minutes. But the care that our predecessors’ newborns required, even two million years ago, was considerably more complicated.
We already know hominid mothers needed help with the birth itself, but once the baby arrived, both mother and child also would have needed additional, serious support just to keep up with the daily migrations of the troop. Attending to her own needs, protecting her baby from predators, and simply managing to keep herself alive would have been a monumental challenge for any australopithecine mother. Not only that, if the birth rate was rising, she would have faced taking care of more than one child at a time. This had to have dramatically shifted the social and sexual dynamics within every troop. Finding mates—truly reliable, helpful mates—would have quickly become a matter of life and death. Maybe new mothers were able to count on the support of other females in the troop (this often happens with chimps), but only up to a point. Life was short and childbearing years few. There would not have been many females available who weren’t already busy with their own offspring.
Sooner or later the most consistent help would have had to come from fathers, not midwives and cousins. This meant females needed to find mates with more to offer than broad shoulders, brains, stamina, and strength. They also had to bring primal versions of patience, honesty, attentiveness, and loyalty to the effort.
We should not underestimate the power of these changes, even though our understanding of how they may have played out is murky. Just as the heightened dangers of the savanna tightened the social fabric among the troop, the need to care for their increasingly helpless young inevitably drew both children and their parents into more emotionally intimate relationships.
We can see part of the evidence of this in the ways we relate with one another today. We are odd among the apes because we are the only ones that are even remotely monogamous. Male gorillas, for example, are polygenous. They have harems of females that they mate with, and jealously guard from competitors. Because the dominant male obviously has a strong set of genes, the system works fairly well, but only because male gorillas don’t have to be strong helpmates. Female gorillas handle raising the young fine without their help.
For their part, chimps are polygamists. Both males and females mate with multiple partners whenever females are fertile. Recently scientists have found that a chimp’s sperm has more mitochondria than human sperm. This actually gives the spermatozoons of different male chimps an increased ability to battle it out with one another in the womb to win the right to fertilize the egg of a female with whom the chimps have mated. (This is selection of the fittest at the most basic level.) Human semen doesn’t have this capacity, and it very likely doesn’t because there never was any overwhelming evolutionary need for it to develop. As a species we mostly stick with one mate (though there are obviously many exceptions), and the b
est theory as to why we do is that evolution favored savanna apes who mated and then worked together to raise and protect the helpless children they were bringing into a dangerous world.
This meant that somewhere in our hominid past something like a family unit began to evolve that also created increasingly complicated social relationships. Tangled strategies were required to figure out which mates would be the best caregivers, and the most reliable and loyal. Complex games of social chess had to have evolved to win the battles for one another’s affection and fidelity. Obviously attraction to a strong set of genes continued to play a central role in selecting mates; this is why we still find big smiles that show off white teeth, and strong, healthy bodies and athletic prowess attractive in others. But now individual traits and personal behavior were also becoming increasingly important to making certain that the species and the troop survived.
The timing of all of these changes is hardly precise, but we can speculate that roughly three thousand millennia had passed since our ancestors and chimps had split off from their common ancestor. By this time multiple species of australopithecines had come and gone. Homo habilis, the first of our direct line, was now a central primate player on the savanna. It was edging increasingly toward humanlike intelligence, behavior, and relationships, all of which would both enable and require finer forms of communication. The first tiny fires of human culture were beginning to glimmer. New creatures were taking shape, and life on Africa’s savannas was about to become more intriguing than ever. Because now that our predecessors had risen up and were standing on their own two feet, something else entirely new was in the process of evolving, and that trait would change them, if possible, even more profoundly than anything else already had.
II
Thumbs
Chapter 3
Mothers of Invention
Now, if some one man in a tribe, more sagacious than the others, invented a new snare or weapon… the plainest self-interest, without the assistance of much reasoning power, would prompt the other members to imitate him; and all would thus profit…. If the invention were an important one, the tribe would increase in number, spread and supplant other tribes.
—Charles Darwin, The Descent of Man
Look at your hand. Hold it up. Flex it. Bend it. Make it act like a puppet. It’s a remarkable piece of engineering. Never before have five digits, fourteen joints, and twenty-seven bones come together in such an interesting and practical way. If you turn it, eight cubelike bones connected by a matrix of tendons in your wrist and forearm enable you to rotate your hand 180 degrees. This makes it possible to do things that animals in the natural world, even if they had the inclination, could never possibly carry off, like swing a baseball bat, pour a glass of milk, play a Duke Ellington piano solo, or paint a portrait.
The fingers of our hands actually have no muscles. They operate by remote control, like marionettes. A web of tendons, anchored in the palm, midforearm, and as far north as the shoulder are the strings that make your digits dance. The whole arrangement provides our hands with an unusually wide range of motion. But the anatomical feature that renders your hand especially special is your first digit, its version of a big toe: your thumb.
One of the great beauties of our thumbs is their position. Whereas our feet forsook the thumblike position of their first digits and evolution straightened them into our big toes, our hands did not. In fact, they went in the opposite direction, building on their former status as feet specialized for climbing and grasping.
This is why our hands still look remarkably like a gorilla’s foot, with our thumbs sitting down below the other four digits, positioned apart, as if they were reluctant to join the rest of the group. Not that this means, however, that thumbs in any way evolved to play second fiddle to the rest of our fingers.
Compared with the thumbs of other primates, ours have acrobatic ranges of motion. Chimp thumbs, for example, can’t rotate in great swirling arcs like a human thumb, and that limits their ability to be the thing that all thumbs secretly long to be…opposable. I say “limits” because, contrary to popular belief, the thumbs of chimps and monkeys are opposable. They just aren’t in the same peculiar way that ours are. What is different is that we can effortlessly swing our thumbs across the palms of our hands to meet our small and ring fingers, the fourth and fifth digits. Nothing like this exists anywhere else in nature. It’s called the ulnar opposition, and this seemingly simple ability gives our hands the power to grasp and grip, turn and twist, manipulate and touch in ways foreign to other creatures. Because of this ability we can pick up and use a hammer or an ax, or turn a stick into a lethal club by cupping it in a position that extends the power of our arm, and, with it, the force of the blow it delivers. It is one thing to flail a stick horizontally for show, like a chimpanzee, another to grip it along the axis of your forearm and bring it down from on high with bone-crushing force.
Ulnar opposition also makes all of the difference between simply grasping a tree branch the way a chimps does when it swings through the forest, and precisely clasping minuscule objects with sweet and exact precision. When picking up something as tiny as a grain of rice, a chimpanzee has to squeeze it between its thumb and the flat of its index finger, like we hold a key or credit card. Such precise use of thumb and finger is a struggle for chimps because they don’t have the musculature and nerve structure we do. We can pick the same grain up using the very tip of our thumb and caress it in the closed circle of our finger, as if we were making the sign for “okay, perfect,” which, in a sense, it is.
These abilities exist because we have developed specialized tendons linked to our thumbs. One, a flexor called the pollicis longus, runs from the thumb’s knuckle all the way to the shoulder. Along with three other muscles, it lets us push and mash things as well as open our hands and spread our thumbs away from our palm, movements that come in handy when operating a joystick, typing on a keyboard, or thumbing in the numbers on a cell phone. But it is also very useful for gripping and manipulating sticks and stones, natural artifacts that our ancestors used to fashion the first tools into axes, spears, and small knives more than two million years ago.
It’s not simply the speed and flexibility of our thumbs, fingers, and hands that make them special. It’s also their extraordinary sensitivity. Crammed within every square inch of our digits are nine thousand hypersensitive egg-shaped, buds called Meissner’s corpuscles, which lay just below the epidermis, our outermost layer of skin. Inside each bud lie coiled nerves that sense and snatch up the signals initiated by whatever we touch and send it to the brain for processing.1 These same nerves are scattered among other particularly sensitive parts of our bodies—our tongues, the soles of our feet, our nipples, penis, clitoris—every erogenous zone. They’re optimized for gathering the finest, most granular pieces of sensual information, and they are why our hands are, as Sir Charles Bell, put it, “so powerful, so free and yet so delicate.”
Without this combination of dexterity and sensitivity, Michelangelo would never have been able to sculpt the face of his Moses, nor Leonardo paint The Last Supper. Horowitz could not even plunk out the most juvenile version of the Emperor Concerto, and Shakespeare would have been incapable of grasping a quill to pen a single word of the thousands he invented for the English language.2
The point here is a subtle one. The physical power and dexterity of our thumbs and hands make them central to our humanity. Their biological evolution literally changed our minds. They enabled us to better manipulate the world around us, and the manipulation of things then came to also mold our minds. This is what prompted novelist Robertson Davies to observe in his book What’s Bred in the Bone, “the hand speaks to the brain as surely as the brain speaks to the hand.” The wirings for creativity, for memory, for emotion, and above all (as we shall see) for language, exist largely because our thumbs came first, and in orchestrating our physical conversations with the world, laid the neural groundwork for the peculiarly human mind that would follow. Our
thumbs are that defining. Without them, we wouldn’t be human. We would be something else.
For the ancestral line of apes that led to us, there would have been a considerable evolutionary advantage in developing thumbs. As they gave up knuckle-walking and spent more time upright, their hands would have been freed to hold more, carry more, throw more, and eventually manipulate and make more. Had early savanna apes not begun walking upright, thumbs would never have evolved. And if they had not, we wouldn’t have either.
…
Hands and thumbs go back in one form or another forty million years ago to prosimians, the line of mammals from which we evolved. But hands, at least the ones to which we have become accustomed, are relatively new. Based on the garbled messages the fossil record has so far provided, science’s best guess is that they reached something like their current, thumb-opposable state a little more than two million years ago. By this time Homo habilis was emerging on the plains of Africa—brainier, faster, and more inventive than the other primates with which it was keeping company.
Exactly which line of australopithecines might have led to Homo habilis remains unresolved. But when this new genus surfaced, with its specialized thumbs, new and interesting events began to unfold. The first and most obvious change was toolmaking. Australopithecines like Lucy and the Taung child were tool users but not toolmakers. Like chimps, they very likely put twigs, bone, grass, and rocks to work as weapons and various other kinds of primeval gadgets, mostly to help gather food.3 But generally scientists agree they did not reshape them into anything sharper or more complex than nature originally intended, partly because they didn’t have the dexterity, and partly because they didn’t have the cerebral horsepower to consider the possibility. They were probably not very different from today’s average chimp.